Cycles render on combination of AMD & Nvidia GPUs in one system
Blend file: 2.7x Cycles benchmark (Updated BMW)

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Testing Methodology:


Load up the .blend file.
Set the "Compute Device" to "OpenCL".
Render twice on AMD R9 290. Record the lowest value.
Set the "Compute Device" to "CUDA".
Render twice on NVidia GTX 460. Record the lowest value.




Results:


AMD R9 290 (OpenCL): 1:43:57
NVidia GTX 460 (CUDA): 3:33:95


Blender cannot use both OpenCL and CUDA simultaneously.